1. Field of the Invention
One disclosed aspect of the embodiments relates to a light emitting apparatus and a driving circuit of a light emitting element and, more particularly, relates to a light emitting apparatus which uses organic electroluminescence device (referred to as an “organic EL element” below) as light emitting elements.
2. Description of the Related Art
An organic EL element is a light emitting element which is formed by sandwiching an organic compound layer between two electrodes, and emits light at brightness matching a current flowing between the electrodes. An application of a light emitting apparatus which uses organic EL elements is a display apparatus in which organic EL elements are arranged in a matrix on a plane. Other applications are an exposure apparatus of an electrophotographic printer in which organic EL elements are provided in a one-dimensional array and a lighting apparatus which emits light of a wide area.
Since a large current flows in an organic EL element upon light emission and a resistance of a conductive line from a power source to a driving circuit of the organic EL element (referred to as a “power line” below) is usually high, a voltage significantly drops along the power line and a voltage applied to the organic EL elements decreases. In a light emitting apparatus in which multiple driving circuits of the organic EL elements are arranged along a power line, the voltage drop results in a difference of a power voltage between an organic EL element close to a power source and an organic EL element far from the power source. This power voltage gradation occurs not only in the power line of an anode side but in a power line of a cathode side of the organic EL element.
Meanwhile, a voltage signal which determines light emission luminance is applied to the driving circuit of the organic EL element through a data line. A current only transiently flows in the data line, and therefore the voltage drops along the data line very little compared to the power line. Hence, the voltage signal is written in the driving circuits irrespectively of the voltage gradation along the power line.
The driving circuit generates a current according to the written signal voltage, and supplies the current to the organic EL element. The current is generated in the driving circuit based on the signal voltage relative to the power voltage. Therefore, even though a signal voltage is uniform, the currents generated in the driving circuits are not uniform due to the gradation of the power voltage.
Japanese Patent Application Laid-Open No. 2006-308845 discloses an embodiment of a driving circuit which first disconnects both ends of a capacitor for holding a signal from the driving circuit, applies a fixed voltage to one end, applies a signal voltage to the other end, and finally connects the capacitor to the driving circuit while maintaining the voltage between both ends of the capacitor. Thus, a current irrespective of the power voltage is generated.
In another embodiment of the patent application, two capacitors are disposed in the driving circuit and a signal voltage is written in a capacitor and a threshold voltage of a driving transistor is held in another capacitor. By connecting the capacitors to the driving circuit, a sum of the signal voltage and the threshold voltage is applied to the driving transistor to generate a current irrespectively of the threshold voltage.
A conventional driving circuit requires two capacitors to cancel variation of the threshold voltage of the driving transistor and cancel influence of variation of the power voltage. Since capacitors occupy a large portion of a pixel area, it is difficult to reduce a pixel size and improve precision of the display.